Subject: Young's double slit experiment questions

1. In Young's double slit experiment, the fringes are alternate and equally spaced.
Why is it so? Does it has something to do with energy distribution? Or it is due
to coherent sources having same phase or constant phase difference. Please explain.
2. In Young's double slit experiment, the two sources are coherent i.e., they have
same amplitude. What would happen if we take two sources of different amplitude?
How will be the interference pattern? Why the coherent sources are necessary?